generator - oscillator
Oscillator circuit is used for providing a
microcontroller with a clock. Clock is needed so that microcontroller
could execute a program or program instructions.
Types of oscillators
PIC16F84 can work with four different configurations of an
oscillator. Since configurations with crystal oscillator and
resistor-capacitor (RC) are the ones that are used most frequently, these
are the only ones we will mention here. Microcontroller type with a
crystal oscillator has in its designation XT, and a microcontroller with
resistor-capacitor pair has a designation RC. This is important because
you need to mention the type of oscillator when buying a
is kept in metal housing with two pins where you have written down
the frequency at which crystal oscillates. One ceramic capacitor of
30pF whose other end is connected to the ground needs to be
connected with each pin.
Oscillator and capacitors can be packed
in joint case with three pins. Such element is called ceramic
resonator and is represented in charts like the one below. Center
pins of the element is the ground, while end pins are connected with
OSC1 and OSC2 pins on the microcontroller. When designing a device,
the rule is to place an oscillator nearer a microcontroller, so as
to avoid any interference on lines on which microcontroller is
receiving a clock.
In applications where great time precision is not
necessary, RC oscillator offers additional savings during purchase.
Resonant frequency of RC oscillator depends on supply voltage rate,
resistance R, capacity C and working temperature. It should be mentioned
here that resonant frequency is also influenced by normal variations in
process parameters, by tolerance of external R and C components,
Above diagram shows how RC oscillator is
connected with PIC16F84. With value of resistor R being below 2.2k,
oscillator can become unstable, or it can even stop the oscillation. With
very high value of R (ex.1M) oscillator becomes very sensitive to noise
and humidity. It is recommended that value of resistor R should be between
3 and 100k. Even though oscillator will work without an external capacitor
(C=0pF), capacitor above 20pF should still be used for noise and
stability. No matter which oscillator is being used, in order to get a
clock that microcontroller works upon, a clock of the oscillator must be
divided by 4. Oscillator clock divided by 4 can also be obtained on
OSC2/CLKOUT pin, and can be used for testing or synchronizing other
Following a supply, oscillator starts
oscillating. Oscillation at first has an unstable period and amplitude,
but after some period of time it becomes stabilized.
To prevent such inaccurate clock from
influencing microcontroller's performance, we need to keep the
microcontroller in reset state during stabilization of oscillator's clock.
Diagram above shows a typical shape of a signal which microcontroller gets
from the quartz oscillator.